Modern internal combustion engine-powered vehicles almost uniformly employ means of exhaust gas treatment prior to exhaust gas discharge to the atmosphere. For non-particulate components of the exhaust, treatment often consists of promoting chemical reactions to manage the composition of the exhaust gases.
This is usually accomplished by directing the engine exhaust gases into an exhaust system incorporating catalytic materials for controlled reaction of the exhaust gases before their release into the atmosphere. Catalytic materials may promote only selected chemical reactions, or selected classes of reactions, so that it is often necessary to incorporate more than one catalyst into the exhaust system. When it is desired to promote a multi-step reaction, a series of catalysts suitably arranged for specific sequential exposure to the exhaust gases may be required.
Diesel engines and other lean-burn engines or power plants are operated at higher than stoichiometric air to fuel mass ratios for improved fuel economy. Such lean-burning engines emit an exhaust gas with a relatively high content of oxygen and nitrogen oxides (NOx). For example, a representative composition contains, by volume, about 6-17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons and 235 ppm NOR. The balance of the exhaust gas is nitrogen and water with small concentrations of sulfur-containing compounds, primarily SO2. It is preferred to reduce the NO gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2), to nitrogen (N2), but these reactions are impeded by the high oxygen (O2) content in the exhaust stream.
Several approaches have been suggested to overcome this limitation, including NOx storage and reduction catalysts and the introduction of ammonia or hydrocarbons into the exhaust stream to facilitate NOx reduction.
However the high oxygen content of a lean burn engine exhaust, 6-17% by volume, renders the reduction of NO2 to N2 more favorable than the reduction of NO to N2. It may therefore be preferred to deplete the exhaust stream of NO and enrich it in NO2 by promoting the oxidation of NO to NO2 in the oxidation catalyst, prior to reduction. But many of the proposed catalysts include expensive noble metals and may be rendered ineffective by the presence of sulfur-containing gases in the exhaust stream.
There is thus a need for less expensive sulfur tolerant oxidation catalysts suitable for operation in a diesel exhaust environment.